In recent years, there have been more opportunities to handle information having large data amount such as an image. On this account, it is necessary to increase recording density of an optical recording medium. In view of this, the following optical recording medium and super-resolution technique are proposed: (i) an optical recording medium on which information is recorded with the use of prepits constituted by concavity and/or convexity shorter than a resolution limit of an optical system of a reproduction device and (ii) a super-resolution technique for reproducing the optical recording medium. Note that an optical recording medium reproduced with the use of the super-resolution technique is hereinafter referred to as “super-resolution medium” or “super-resolution optical recording medium”, and a conventional optical recording medium, i.e., an optical recording medium which does not require use of the super-resolution technique and on which information is recorded with the use of prepits longer than the resolution limit of the optical system of the reproduction device is hereinafter referred to as “normal medium” or “normal optical recording medium”. Further, note that the resolution limit of the optical system is determined based on (i) a wavelength of a laser beam with which the reproduction device irradiates a medium and (ii) a numerical aperture of the optical system.
Examples of such a super-resolution medium include optical recording media disclosed in Patent Literatures 1 and 2, respectively.
According to the super-resolution medium disclosed in Patent Literature 1, a thermochromic layer whose optical properties such as transmissivity varies depending on temperature is provided on a surface of a reflecting layer on which surface laser beam is incident. In a case where the thermochromic layer is irradiated with a laser beam having certain power, a super-resolution effect that a spot of the laser beam goes into a pseudo-reduction state is produced. Thus, the thermochromic layer functions as a mask layer. Temperature distribution is caused in the spot on the mask layer due to light intensity distribution, and transmissivity distribution is caused due to the temperature distribution. For example, in a case where the mask layer is made of a material whose transmissivity becomes higher as the temperature rises, only parts of the mask layer which have high temperature are high in transmissivity. Thus, the spot formed on the surface of the reflecting layer is brought into the pseudo-reduction state. This allows reproduction of a prepit shorter than a resolution limit of an optical system. The technique disclosed in Patent Literature 1 can be applied not only to a rewritable optical recording medium but also to a read-only optical recording medium.
Further, according to the super-resolution medium disclosed in Patent Literature 2, a film layer (referred to as “function layer”) made of a thin metal film or the like is provided on a substrate on which information is recorded with the use of prepits which are concavities and/or convexities. At present, it is unknown how a super-resolution technique used for the super-resolution medium of Patent Literature 2 works. However, it is possible to reproduce a prepit shorter than the resolution limit of the optical system in a case where the temperature of the function layer is changed by irradiating the function layer with a laser beam having higher power than usual.